Touch-sensing systems (“touch systems”) are in widespread use in a variety of applications. Typically, the touch systems are actuated by a touching object such as a finger or stylus, either in direct contact, or through proximity (i.e. without contact), with a touch surface. Touch systems are for example used as touch pads of laptop computers, in control panels, and as overlays to displays on e.g. hand held devices, such as mobile telephones. A touch panel that is overlaid on or integrated in a display is also denoted a “touch screen”. Many other applications are known in the art.
There are numerous known techniques for providing touch sensitivity, e.g. by incorporating resistive wire grids, capacitive sensors, strain gauges, etc into a touch panel. There are also various types of optical touch systems, which e.g. detect shadows cast by touching objects onto a touch surface, or detect light scattered off the point(s) of touching objects on a touch panel.
One specific type of optical touch system uses projection measurements of light that propagates on a plurality of propagation paths inside a light transmissive panel that defines a touch surface. The projection measurements thus quantify a property, e.g. power, of the light on the individual propagation paths, when the light has passed the panel. The light propagates inside the panel by total internal reflection (TIR) against the touch surface, such that objects on the touch surface causes the propagating light on one or more propagation paths to be attenuated, commonly denoted FTIR (Frustrated Total Internal Reflection). For touch determination, the projection measurements may be processed by simple triangulation, or by more advanced image reconstruction techniques that generate a two-dimensional distribution of disturbances on the touch surface, i.e. an “image” of everything on the touch surface that affects the measured property. Examples of such touch systems are found in U.S. Pat. Nos. 3,673,327, 4,254,333, 6,972,753, 7,432,893, US2006/0114237, US2007/0075648, WO2009/048365, US2009/0153519, WO2010/006882, WO2010/064983, WO2010/134865 and WO2012/105893.
The prior art suggests several different approaches for introducing the light into the panel and for detecting the light downstream of the touch surface. For example, U.S. Pat. No. 7,432,893 proposes coupling light into the panel via revolved prisms that are attached to the rear surface of the panel, and detecting the light at photodetectors that are directly attached to the front surface of panel. In WO2010/064983, light is coupled into and out of the panel via the edge surface that connects the front and rear surfaces of the panel, or via wedges that are attached to the front or rear surface of the panel. In WO2012/105893, a sheet-like microstructured element, e.g. a tape of light transmissive material, is provided on the front or rear surface of the panel for coupling light into and out of the panel.
One challenge when designing an optical touch system of this type is to enable consistent touch determination despite the fact that the detectors need to detect small changes in weak optical signals in presence of potentially significant interferences that affect the reliability of the optical signals. One such interference is caused by ambient light, e.g. from sunlight or residential lighting, that may impinge on the detectors and influence the optical signals. Another interference is caused by accumulation of contamination on the touch surface, such a fingerprints, drops of saliva, sweat, smear, liquid spills, etc. The contamination will interact with the propagating light and cause changes to the optical signals that may be difficult to distinguish from changes caused by “true objects”, e.g. objects that are actively manipulated in contact with the touch surface.
In touch-sensitive devices, there is also a general trend to avoid attaching components to the front surface. These components may form a frame around the touch-sensitive region and thereby reduce the ratio of the active area (the surface area that is available for touch interaction) to the total surface area of the touch-sensitive device. Furthermore, if the components protrude from the front surface of the panel, it may be necessary to provide a bezel at the perimeter of the panel to protect and hide the components and possibly any wiring connected to the components. Given the nature of user interaction with touch-sensitive devices, such a bezel may disrupt the user experience and even prevent certain types of interaction. The bezel may also cause dirt and other contaminants to accumulate in the area where the bezel joins the panel. To overcome this problem, it is desirable to design touch systems for flush mount of the panel in the supporting frame of the touch-sensitive device, i.e. such that the front surface of the panel is essentially level with the surrounding frame material. This is also known as “edge-to-edge”.
In aforesaid U.S. Pat. No. 7,432,893, the impact of ambient light is reduced by attaching the photodetectors to the front surface, such that the photodetectors face away from the ambient light that enters the panel through the front surface. This solution requires a significant bezel to hide and protect the photodetectors and the associated wiring. U.S. Pat. No. 7,432,893 also proposes to intermittently measure ambient levels at the photodetectors and compensate the respective projection measurement for the measured ambient level.
The influence of contamination may be handled by dedicated signal processing that actively estimates the influence of contamination over time and compensates for this influence, e.g. as disclosed in WO2011/028169, WO2011/049512 and WO2012/121652.
However, in view of the weak optical signals and small attenuation caused by touching objects, there is room for further improvement when it comes to increasing the robustness of the touch system to ambient light and contamination on the touch surface.
In the field of LCD display technology, which is outside the field of touch-sensitive systems, it is known to couple light from LEDs into thin light guide panels as part of so-called backlights (BLUs, Backlight units) for LCD displays. The light guide panel is located behind the LCD and is configured to emit light from its top surface to uniformly illuminate the rear side of the LCD. One approach for coupling the LEDs to the light guide panel is proposed in the article “Injecting Light of High-Power LEDs into Thin Light Guides”, by Cornelissen et al, published in Proc. SPIE 7652, International Optical Design Conference 2010, pp 7652121-7652126, 2010. According to this approach, the top surface of the LED is modified to have a rough surface behaving like a Lambertian reflector. A dielectric multilayer filter is deposited on the bottom of the light guide panel, and the top surface of the LED is optically coupled to the filter by a silicone adhesive. The filter is optimized to only transmit light emitted from the LED at angles larger than the critical angle at the light guide-air interface. The purpose of the multilayer is thus to only transmit light that can propagate in the light guide. The light emitted at smaller angles is reflected back toward the rough LED surface where it is subsequently recycled by reflection and redistribution. Further elaboration on this incoupling approach is given in the article “Dielectric multilayer angular filters for coupling LEDs to thin light guides”, by Mu et al, published in Proc. SPIE 8170, pp 8170011-81700110, 2011.